1. Field of the Invention
The present invention relates to a thermal image transfer recording medium comprising a support and a thermofusible ink layer formed thereon, which is excellent in preservability and printing performance at low temperatures, and more particularly to a thermal image transfer recording medium which is particularly suitable for use as a bar code recording medium, free from the problem of the peeling of the thermofusible ink layer off the support at low temperatures, and capable of yielding clear bar codes when printed in the direction at an angle of 90.degree. with respect to the transporting direction of the recording medium.
2. Discussion of Background
Recently a thermosensitive image transfer recording system using a thermal head is widely used because of the advantages that it is noiseless, the apparatus for use in the system is relatively inexpensive and can be made small in size, the maintenance is easy, and printed images are stable in quality.
Representative examples of thermosensitive image transfer recording media for use with such a thermosensitive image transfer recording system are as follows:
(1) A thermosensitive image transfer recording medium comprising a support and a thermofusible ink layer which comprises a coloring agent and a binder agent and is directly provided on the support.
(2) A thermosensitive image transfer recording medium comprising a support and a two-layered thermofusible ink layer provided on the support, comprising a first ink layer provided on the support side and a second ink layer overlaid on the first ink layer. The first ink layer essentially consists of a wax component, while the second ink layer essentially consists of a coloring agent and a binder agent.
Generally many binder agents for use in the thermofusible ink layer in the thermosensitive image transfer recording medium (1) and for use in the first ink layer of the thermosensitive image transfer recording medium (2) comprise as the main component a wax with low plasticity. Therefore the adhesion of such ink layers with a support, for example, a support made of a resin such as polyethylene terephthalate (PET), which is mostly widely used, is so poor that there is the disadvantage that such thermofusible ink layers easily peel off the support when some mechanical force is applied. This disadvantage is particularly conspicuous when such thermosensitive image transfer recording media are placed at low temperatures, for example, at 5.degree. C., and at low humidities, for example, 10-50% RH.
In an attempt to overcome the above disadvantages of the conventional thermosensitive image transfer recording media, for example, the following methods have been proposed: making the surface of the support irregular to increase the contact surface area between the ink layer and the support as disclosed in Japanese Laid-Open Patent Application 58-16889; and providing an intermediate layer comprising a cellulose resin or polyester resin between the support and the ink layer as disclosed in Japanese Laid-Open Patent Applications 59-165690 and 60-54894.
These methods, however, have the shortcoming that the production cost is high because of the complicated production process.
Furthermore, as mentioned above, in the conventional thermosensitive image transfer recording media, many binder agents for use in the thermofusible ink layers, for example, for use in the thermofusible ink layer in the thermosensitive image transfer recording medium (1) and in the first and second ink layers of the thermosensitive image transfer recording medium (2), comprise as the main component a wax with low plasticity. Therefore, the adhesion strength of such ink layers to the support is so weak that even non-heated ink layer portions, which are not heated by a thermal head, tend to peel off the support in the course of printing serial bar codes by a bar code printer, thereby making printed bar codes significantly unclear. This makes it impossible to read the printed bar codes by a bar code scanner.
Furthermore, the printed bar codes do not have a sufficient abrasion resistance for use in practice. More specifically, it occurs very frequently that the printed bar codes become illegible when frictioned, for instance, with a corrugated fiberboard.